1. Field of the Invention
The present invention is concerned with implantable orthopedic devices that prevent the loss of bone, stimulate the growth of bone tissue, and enhance fracture healing.
2. Description of the Prior Art
Osteoporosis and other degenerative diseases, including bone trauma, often require the surgical insertion of a prosthetic implant to replace the damaged tissue. The implant must be biocompatible and have the same structural integrity as the tissues it replaces. Replacement of the joints in the hip may require the use of artificial components, in the form of a prosthetic implant inserted into the femur and corresponding acetabulum forming the replacement joint. The knee joint is replaced with two corresponding components, inserted into the femur and tibia, forming the knee joint. These prosthetic implants have been made of stainless steel, cobalt-chromium-molybdenum alloys, titanium alloys, ceramics, and other materials. In order for these materials to be effective as a long term replacement of the joint, the prosthetic implant must provide a good substrate into which the surrounding bone can mechanically integrate and achieve long-term stability. Though much research has been devoted to this concept, these materials fail to achieve excellent mechanical integration and the failure rate of these devices remains high.
Many prosthetic implants have been developed to address implant failure by attempting to enhance the mechanical integration between the implant and the bone. Two such methods stabilizing implantable prosthetic devices are the use of polymethyl methacrylate (PMMA) bone cement and the application of a porous coating on the surface of the implant. The PMMA bone cement can cause harmful side effects and can cause osteolysis which leads to implant failure. In addition, the body may, in response to the implant, form a fibrous tissue capsule surrounding the implant, which causes loosening and failure of the prosthetic device. Failure can also occur if the bone cement fractures or weakens. The technique of porously coating the prosthesis with sintered beads was developed for press-fit implants. These implants do not require cement and achieve a scratch fit against the bone. The bone grows in a three-dimensional nature around these beads, which stabilizes the implant. The porous coat can be applied to either the entire surface, or to proximal sections of the implant so as to facilitate mechanical bonding. The porous coat may also vary according to the size of the sintered bead, which is believed to stimulate different amounts of bone bonding to the implant. This method has had short term success in achieving a mechanical interface with the surrounding bone tissue; however, this application fails in the long-term because of stress shielding, an uneven transfer of weight-bearing stresses to the bone. The porous coat also does not induce a uniform attachment of the bone to the implant substrate which often causes failure of the implant. Lastly, the rate at which the surrounding bone tissue grows into the implant remains slow, which calls for extensive recovery time and there is an increased potential to develop complications.
Bone responds to varying applied stresses and will remodel to best suite those stresses, as recognized by Wolff in 1892. A typical implant reduces stress transfer to the bone in proximal areas around femoral hip implants, while increasing stress transfer in distal areas. This leads to hip pain and eventual bone fracture.
Bone has the ability to detect an externally applied load as a mechanical strain and respond to that strain via tissue remodeling. When stress is applied to a bone in a body, certain materials in bone tissue polarize and exhibit piezoelectric properties. A voltage develops along the loading axis, which stimulates osteoblast cells in the bone matrix to form new bone. At the same time, osteoclast cells break down bone which may account for the similarity in the rates of new bone formation and old bone breakdown, which may maintain bone distribution and bone density at a substantially uniform level. When an orthopedic femoral hip implant is placed in the body, the natural loading environment of the body changes as the regions proximal to the implant receive decreased levels of stress as compared to distal regions. The ultimate result of this phenomenon may be the failure of the implanted prosthesis which will necessitate further surgery.
Attempts have been made to chemically bond implants to bone tissue by the use of a coating of hydroxyapatite or other similar calcium containing chemical compounds. The chemical bonding stabilizes the implant by filling in gaps and providing a cementing effect. The use of chemical compounds does address the problem of protecting the implant from stresses which can cause the implant to loosen and fail. Piezoelectric materials have been used in connection with implants to stimulate bone growth. U.S. Pat. No. 6,143,035 utilizes surface applied piezoelectric elements that are attached to a hip implant with the outer surface polarized to have a negative charge. The piezoelectric element is placed on the medial side of the shoulder or neck of the implant where it will undergo compressive strain. Other embodiments used piezoelectric elements that were mounted on a bone in a position remote from the area where bone growth was desired. Electrical conductors were implanted to conduct the remotely generated current to the site where an implant is placed.
U.S. Pat. No. 6,571,130 discloses a biocompatible piezoelectric element for use as an implanted sensor. U.S. Pat. No. 6,447,542 discloses an implantable porous member that has pores which may be filled with a piezoelectric composition which has the ability to stimulate cell growth by generating an electrical field in response to mechanical stress.
The form and function of the musculo-skeletal system is closely related to the forces acting in its components. Fracture treatment by means of intramedullary nails is an accepted and widely used method of treating transverse and short oblique, axially stable fractures of the femoral diaphysis. However, complications may arise during fracture healing and non-unions, delayed unions, and mal-unions have been reported as well. The introduction of the interlocking nail has allowed treatment of comminuted femoral fractures, because limb rotation and length can be maintained but healing is still a problem in many cases. In femoral nailing, two treatment modalities can be discerned: Static locking connects the implants with the main proximal and distal bone fragments, securing their relative position and orientation. Dynamic locking with fewer connecting screws should prevent rotation while allowing dynamic interfragmentary compression. Experimentally, the beneficial effects of electricity in bone healing have been demonstrated in long bone fracture models. In 1957, Fukada and Yasuda (J.Physiol Soc Jpn 12:1158-1162(1957)) showed that a continuous current of 1 μA over 3 weeks produce new bone growth in rabbit femora. Direct current delivered via electrodes in long bones results in osteogenesis around the negative electrode and resorption around the positive electrode. Ultrasound emitting devices and capacitive coupling devices have been shown to reduce the rates of delayed unions and non-union fractures. Therefore, the present invention also includes the use of direct current stimulation of bone growth by imbedding piezoelectric elements into intramedullary rods and interlocking nails.
The prior art does not disclose the concept of providing below the surface mounted, discrete piezoelectric elements that are positioned in an implantable device in such a manner that they are separated from one another and extend slightly from the surface of the implant, are slightly below the surface of the implant or are flush with the surface of the implant. The use of spaced apart individual piezoelectric elements is believed to make possible the provision of more effective electrical fields for the stimulation of bone growth by locating the piezoelectric elements where the bone growth is more important to the long term success of the implant. For example, in the case of a hip implant, bone growth at the upper end of the femur is more important than in other areas contacted with the implant because of the higher stresses that are applied to the upper end of the femur.